This invention relates to a nonpressurized solar heating system which utilizes a heat transfer fluid having a high boiling point and a low freezing point.
Conventional solar heating systems typically include some type of solar collector array through which water or heat transfer fluid is circulated for heating. The solar collector array absorbs the sun's rays to heat the water or fluid flowing therethrough and then supplies the fluid to a heat exchanger where the solar heat is extracted for use or storage. From the heat exchanger the heat transfer fluid flows to an expansion tank and from there it is pumped back to the solar collector array.
If water is employed in the solar heating system, then when the system is not in use, the water must be drained from the system to avoid freezing or boiling and the consequent damage to the solar collector array or system. To avoid freezing without the need for frequent draining, it has been proposed that an "antifreeze" fluid be used in the solar heating systems. However, systems using antifreeze fluids encounter the problem of the fluid boiling when the system is not in operation. This occurs because fluid which is static in a solar collector array exposed to the sun increases dramatically in temperature to the point where it boils. One approach to diminishing the boiling problem is to supply the fluid to and maintain the fluid in the system under pressure. Although this does serve to inhibit boiling to a certain extent, it also introduces other problems, such as the system's gradual but inevitable pressure loss, the need of pressure control equipment in the system, and the need of specialized equipment to fill the system under pressure. Even with pressurized systems, the antifreeze fluid may still tend to boil uner high solar energy conditions and so to avoid this, attempts are usually made to operate such systems constantly at all times of solar energy availability. This is more costly, less reliable, and causes greater wear on the system. It also causes overheating of the solar storage system in many circumstances.
Exemplary prior art systems are disclosed in U.S. Pat. Nos. 4,027,821, 4,109,639, 3,970,069, 4,052,000, 4,121,566 3,986,489, 4,119,087, 4,207,866, 4,133,338, 4,191,329, and 4,019,495.